In addition to propulsion, aircraft main engines also provide shaft power through a gearbox to drive electric generators, hydraulic pumps, and fuel pumps. They also provide pressurized air for the environmental control system, which uses this air to cool or heat, ventilate, and pressurize the aircraft cabin. The extraction of pressurized air and shaft power from the main engines is referred to as parasitic loss, and must be compensated for by increasing the fuel flow to the main engines. It has long been a goal of aircraft and aircraft engine designers to minimize or eliminate these parasitic losses, and thereby improve the fuel efficiency of the main engines. In particular, it has been recognized that the electrical generators and environmental control systems are the two largest sources of parasitic losses.
When the aircraft is on the ground and the main engines are shutdown, many types of aircraft employ an auxiliary gas turbine engine to supply pressurized air for environmental control systems or main engine starting, and shaft horsepower to drive accessories such as an electric generator. Recent technological advances now enable these engines to be started and operated inflight in an emergency such as a main engine shutdown, however, under normal inflight operating conditions the burden of providing electricity and pressurized air still falls on the main engines.
Cronin, U.S. Pat. No. 4,494,372 discloses a multi-role primary/auxiliary power system for an aircraft that provides electrical, mechanical, and air conditioning power as well as main engine starting functions. Although this system eliminates the parasitic loss due the extraction of air from the main engine, it still needs mechanical power from the main engine to provide electrical power to the aircraft. Similarly, Cronin, U.S. Pat. No. 4,684,081 discloses a multifunction power system that combines an auxiliary power unit, environmental control system, engine start system, and emergency power system for an aircraft. This system also does not require pressurized air from the engine, however, in the flight cooling mode electrical power is extracted from the main engines to power the system.
Christoff, U.S. Pat. No. 4,503,666 discloses an integrated auxiliary power and environmental control unit having a power turbine and a cooling turbine mechanically coupled to a single compressor. However, inflight pressurized air from the main engines is still needed to operate the environmental control unit.
Accordingly, there is still a need for a an aircraft subsystem that eliminates parasitic losses by providing both electricity and conditioned air to the aircraft independently of the main engines.